Specialists worked into the night in the world's first bid to separate adult twins joined at the head. The delicate procedure at Raffles Hospital in Singapore is expected to last at least 48 hours with Ladan and Laleh Bijani seated at a special operating table the entire time.

The unprecedented surgery has been progressing since the 29-year-old Iranian twins were wheeled into the operating theater early Sunday. Dr. Prem Kumor Nair, the hospital's spokesman, said anesthesia has been administered and special lines to monitor the sisters' vital signs are in place.

Despite the risks of death or severe disability, Ladan and Laleh were in good spirits, according to Dr. Nair. He said a group of seven Iranian friends accompanied the women to their brain scans prior to the surgery and the sisters were joking with their companions when wheeled out of the radiology department.

The 28-member team of specialists from Singapore, the United States, France, Japan, Switzerland and Nepal have been particularly concerned about a shared blood vessel, which allows blood to flow out from their brains.

Dr. Nair said vascular surgeons took a large vein from the thigh of one of the sisters, which will be used as another brain vessel. The women's brains are in a shared cranial cavity and the neurosurgical separation of the brains will be tackled next.

The plight of Ladan and Laleh, who have remained steadfast in their decision to accept the high risks of death or severe disability rather than going on living conjoined, evoked a tribute from Iranian President Mohammed Khatami. According to the state-run news agency IRNA, he said the entire country is praying for the success of the operation. The president also expressed the hope Laleh and Ladan will be back in their homeland as soon as possible.

Professor Walter Tan, the hospital's medical director, said the length of their stay in Singapore depends upon the outcome of the surgery and the aftermath. "The immediate post-op, obviously, will be done here. ... They will not be transferred if they are not fit," he said.

The Nepali community in Singapore rallied enthusiastically to the hospital's call for blood donors and the doctors have all they need in anticipation of heavy blood loss. Other Muslims also were big contributors.

The physicians, headed by Dr. Keith Goh, are optimistic the operation will be successful because the brains are anatomically intact.

Dr. Goh, who headed the team that separated 10-month-old Nepalese twins in April, 1991, has noted developing brains in children have an ability to reorganize and adapt. While adult brains are less adaptable, he said there is evidence that new neuro-reconnections do occur in adults.

Ladan and Laleh describe themselves as opposites. Should all go as they hope, Ladan would like to pursue law while Laleh becomes a journalist.